What's a good resource for getting familiar with reinforcement learning?
To the good answers here, I would add


A brief overview of RL: Most essential concepts in one place.
Another brief overview, in presentation format.
Ben Recht's An outsider's tour of RL is pretty comprehensive and accessible.
The Bellman equations: central to the whole RL theory.
Policy gradients explained by Andrej Karpathy (mentioned in other answers as "pong from pixels", this is the link).


These barely scratch the surface of RL, but they should get you started.